This invention relates to a battery circuit adapted to control connection between a main circuit of an IC (integrated circuit) memory card or, more specifically, a random access memory card and a battery incorporated in the card.
FIG. 3 shows a block diagram of the construction of a conventional random access memory card (hereinafter referred to as "RAM card") which is described in "IC Memory Card Guideline" published in Sept., 1986 by Nippon Denshi Kogyo Shinko Kyokai. Referring to FIG. 3, a RAM chip section 2 is ordinarily constituted by a plurality of RAM chips. An interface connector 1 is provided for connection to an external unit. A lower address line 6 and a data line 7 which extend from the interface connector 1 are connected to respective RAM chips in the RAM chip section 2. A control line 4 and an upper address line 5 which also extend from the interface connector 1 are connected to a chip selection circuit 3 for selecting a designated one of the RAM chips in the RAM chip section 2. The control line 4 further extends from the chip selection circuit 3 and is connected to respective RAM chips of the RAM chip section 2. Upper addresses are addresses for designating one of the RAM chips to be selected while lower addresses are addresses provided in each RAM chip. An external power supply line 8 indi.cated with Vcc and a ground line 9 indicated with GND extend from the interface connector 1. Power is supplied through the lines 8, 9 from a power source (not shown) to the RAM card via the interface connector 1 at a voltage Vcc. The lines 8, 9 are connected to a power supply control circuit 10. A power supply line lla which extends from a data backup battery 11 incorporated in the RAM card to the circuit 10. Power from the battery 11 keeps data stored in the RAM chip section 2 during the time when the RAM card is detached from the IC card terminal and when it cannot be supplied with power from the outside. The power supply control circuit 10 supplies power from the external power source or the battery 11 to the RAM chip section 2 and to the chip selection circuit 3 via a power supply line 12 indicated with V and a ground line 13 indicated with GND.
FIG. 4 shows one of simplest examples of the arrangement of the power control circuit 10 and the data backup battery 11 of the RAM card shown in FIG. 3, in order to explain fundamental functions thereof. In FIG. 4, components identical to those shown in FIG. 3 are indicated by the same reference characters. There are provided a diode 14 for preventing a battery current leak from the battery 11 to the outside of the RAM card and a diode 15 for preventing a current from flowing into the battery 11 via the outside power supply line 8. The diodes 14 and 15 constitute the simplest example of the control circuit 10. The battery 11 is incorporated in the RAM card as shown in FIGS. 3 and 4 in order to maintain data stored in volatile RAM chips provided in the RAM card. The battery 11 may ordinarily be a lithium battery having an output level of about 3 V and having stable characteristics during long-time use, because the holding voltage necessary for backup operation for maintaining data in the RAM chips is at least about 2 V.
FIGS. 5 and 6 indicate battery characteristics. FIG. 5 shows discharge-load characteristics of a lithium battery with respect to the battery voltage (ordinate) and the duration (abscissa), and shows that the output voltage is constantly maintained at about 3 V for a certain initial duration time substantially irrespective of the number of memory chips, that is, the load corresponding to the whole of the RAM chip section 2 (e.g., 100 .OMEGA., 50 .OMEGA., or 30 .OMEGA.) but it abruptly decreases after a certain time has passed. FIG. 6 shows the relationship between the battery operation current (ordinate) flowing out of the battery and the discharge life (abscissa), and shows that if the battery operation current decreases to half, the discharge life, namely, the available time becomes doubled.
FIG. 7 shows the relationship between a current (ordinate) ordinarily called a backup current necessary for retention of data in the RAM chips and a voltage ordinarily called a holding voltage (abscissa), and shows that if the holding voltage applied to the RAM chips is increased by times, the backup current is also increased by 1.5 times.
Recently, RAM cards have been designed to incorporate an increased number of RAM chips so as to constitute a memory of a larger capacity. The battery life is thereby reduced and, therefore, the majority of recent RAM cards are designed to be of the replaceable battery type. In the case of a type of large-capacity RAM card currently used, the battery life is about 0.7 year. If the life of batteries used in such RAM cards is increased, the user can use the card without anxiety.
The thus-constructed conventional large-capacity RAM card must be designed as the replaceable battery type. A RAM card of the replaceable battery type must be provided with a battery holder having a complicated structure together with a strong outer casing. Such a RAM card tends to malfunction due to failure in contact between the battery and a terminal or breakage of the terminal. In addition, an extra terminal for monitoring the reduction in the battery voltage to constantly observe the same to know the time when the battery should be replaced.